This invention relates to a method for manufacturing a composite material, in particular a ceramics or the like containing composite material, as well as to a composite material.
The use of ceramics in applications where conventionally metals were used has recently increased markedly. Reasons for this are, for instance, various clearly improved material properties, such as wear-resistance, hardness, corrosion-resistance, modulus of elasticity, dimensional stability, chemical resistance and heat-resistance. One of the important disadvantages of the use of ceramic materials, however, is that they are relatively brittle. Further, their resistance to thermal shocks is poor. Moreover, good ceramic composites are relatively expensive. As a consequence, the possible areas of application of ceramic composites are limited.
In order to avoid these disadvantages, a number of proposals have been made for the manufacture of such composites with improved properties.
European patent application 0,378,500 describes a method for manufacturing a metal-infiltrated composite material which comprises reaction products of the metal with boron and/or carbon. To that end, from a boron door material and a carbon donor material, a self-supporting intermediate of a relatively high porosity is formed, whereby bonds between the material particles are effected, for instance during sintering of the intermediate. Thereafter the intermediate is contacted with a molten parent metal, in a manner and for a time suitable to obtain a reactive infiltration. The space existing between the mutually bonded particles is thereby filled with the metal and the whole is maintained at a suitable temperature for a suitable time, in such a manner that at least chemical bonds are formed between boron and metal, carbon and metal and boron, and/or carbon and metal. As a result of the reactive infiltration, a composite material is formed with mutually bonded particles with residual metal between them.
In this method, bonds between the different particles are effected, partly prior to the infiltration. As a result, a base product with a relatively high density and relatively coarse particles is formed, whereby the product does not acquire a completely open porous network. Any closed porosity present in the product is not removed and therefore not filled with metal during the infiltration. The particles are not entirely surrounded by the metal, so that no fully continuous matrix is obtained in which the particles are embedded. Moreover, the properties of the starting materials change considerably as a result of the chemical reactions.
U.S. Pat. No. 4,879,262 describes a method for manufacturing composites and in particular boron-containing composites, using combustion synthesis of boride compounds and composites. To that end, a suitable mixture of at least a first, B4C rich component and a second, B4C/TiB2 rich component is composed and heated such that a maximum inclusion of the relatively light B4C into the relatively heavy B4C/TiB2 is obtained, whereafter a self-sustaining combustion is effected in the mixture, such that a densification of the matrix arises as a result of the chemical reaction. The densification is not maximal, so that a porous structure is left. Thereafter the porous composite obtained is infiltrated with liquid metal, for instance aluminum. As a result, a composite of a relatively high density is formed. It is noted that in this way other composites can also be obtained, provided a self-sustaining combustion front can be generated therein.
This method can only be used with specific combinations of starting materials, while moreover heating prior to the combustion is required in order to obtain a good densification. Further, a relatively coarse division of the particles is obtained, while the particles will frequently be in mutual abutment. Any porosity present, which may or may not have arisen during the reaction, is not prevented, reduced or removed in this known method. No completely open porous network is formed, so that the particles cannot be completely surrounded by the metal. Moreover, as a result of chemical reactions that occur, the properties of the starting materials change.
European patent application 0,207,371 describes a method for manufacturing composites, in which powders are dynamically densified to a very high relative density. The shock induced by an explosive and/or strike plate in this method should be so high that exothermic sintering of the powders occurs. Chemical bonds and possibly plastic bonds between the powder particles are thereby formed, so that a closed network is obtained of, for instance, metals, oxides and the like, and a substantially full density.
In this method a very strong shock should be induced, in such a manner that the starting powders enter into an exothermic chemical reaction. As a result, the composition, and hence the chemical and mechanical properties, of the mixture changes. Moreover, a continuous network of ceramic particles fixedly bonded to each other is obtained, which particles are therefore not embedded in another material which forms a continuous matrix. As a result, the material obtained does not have an optimum resistance to, for instance, thermal shocks and it is insufficiently tough. Moreover, only starting powders capable of entering into the desired exothermic reactions with each other can be used.
The object of the invention is to provide a method of the type described in the opening paragraph hereof, in which a relatively brittle, powdered material, preferably ceramic particles, in the composite are substantially, at least practically completely embedded in a matrix of a second material which through infiltration is introduced into a product formed by the relatively brittle material, with a completely open porous network. To that end, the method according to the invention is characterized by the steps of:
dynamically densifying an amount of granular or powdered relatively brittle material or a mixture of one or more of such materials;
whereby
the material or mixture of materials is densified in such a manner that a continuous porous product is obtained;
and infiltrating this with a second material,
whereby
after infiltration the brittle material particles are embedded in a continuous network of the second material.
In this description xe2x80x9cdynamic densificationxe2x80x9d, also referred to xe2x80x9cexplosive compactionxe2x80x9d or xe2x80x9cshock densificationxe2x80x9d, is understood to mean: densifying a material using shock waves. For a review, reference is made to R. Prxc3xcmmer, Ber. Dt. Keram. Ges.50, pp. 75-81, (1973).
By dynamically densifying the brittle material in accordance with the invention, a relatively highly dense product which has a fully open porous structure is obtained. That is to say, the particles are densely stacked without forming any fixed bonds between them. Thereafter, through infiltration, the porous network can be filled with a second material, for instance liquid metal or like material. Preferably, capillary infiltration is used. Owing to the product formed having an open porous continuous network, the particles can be entirely circumfused with metal. Thus a continuous matrix of the second material is obtained, in which the brittle particles are completely embedded, as a result of which the desired properties are obtained.
In a preferred embodiment, the method according to the invention is characterized in that the brittle material is predensified prior to the dynamic densification. Such predensification can occur in different ways, for instance by pressing the starting powders by using vibration techniques, by sludge densification and the like, or by combinations of different techniques. In some cases, pouring the brittle material into a die may already provide for a suitable predensification.
During the dynamic densification, in particular the pressure and the pulse duration that occur in the predensified product supply too little energy for bonds to form between the individual powder particles. On the other hand, a high powder density is obtained, of, for instance 80 to 90% of the theoretical maximum density (TMD). When in this description the term xe2x80x9cdensityxe2x80x9d is used the density based on the TMD is meant, unless stated otherwise. This high density is obtained although the (ceramic) starting material is brittle. During the dynamic densification, as a result of a shock wave passage, very high stresses arise at the points of contact of the different particles. As a result, a large part of the particles, and preferably substantially all particles, will break. In accordance with the invention, the subparticles thus formed will come to lie closer to each other, so that the density increases. Moreover, closed pores, if any, are opened as a result of the particles breaking and cracking. Thus a substantially completely open porous network of very finely divided channels is formed. In fact, as a result of the high stress concentration around closed pores, precisely the particles adjacent thereto will break first. Accordingly, during the dynamic densification, from powders with relatively large particles, a powder of a high density is obtained, consisting of very fine, irregularly shaped powder particles having between them a network of irregularly shaped, widely branched and continuous channels. The porosity is substantially entirely open.
By infiltration, the entire network of channels is filled with a second material. Preferably, this second material is relatively tough with respect to the brittle material. By way of example, a metal such as aluminum is mentioned. As a result of the rupture of the particles during the dynamic densification, the relatively brittle (material) particles acquire a very large total surface that comes into contact with the second material. Since the channels fill up substantially completely with the second material, the particles cannot move apart without thereby transmitting forces to the binding, relatively tough, second material. As a result, the composite acquires a toughness that approximates the toughness of the binding material. Moreover, the binding, infiltrated material, in particular when metals are used, provides for a good thermal conductivity, so that the occurrence of high stress concentrations in the composite material during extreme heating and cooling is prevented. As a result, thermo-cracks, certainly on a macroscale, are prevented. This means that the composite material obtained is tough and has a high thermoshock-resistance.
Upon heating of the composite material to above the melting temperature of the infiltrated material, this infiltrant may partly egress from the channels and form drops on the heated surface of the composite material. Upon heating to above the boiling temperature, it will form a gaseous cushion thereon which will prevent further heating of the surface by conduction. As a result, the melting of the relatively brittle (material) particles themselves is counteracted.
Since the infiltrated material is located in relatively narrow, highly tortuous channels and fills them up entirely, it is largely retained in the channels in liquid condition as well, as a result of inter alia capillary attraction. Owing to the large surface area/volume ratio of the brittle material particles themselves, a strong, tough bond is formed between the powder particles by the infiltrated material. As a result, the composite thus obtained has a high tensile strength.
If as a second material a different material is used which, for instance, has a toughness comparable with the brittle material, such as another ceramic material or a plastic such as a monomer or oligomer which after infiltration is converted into a (co)polymer, then a composite is obtained with other, favorable properties, for instance extreme impact resistance in addition to a low weight and high density. In that case, the polymerization can preferably be initiated from the outside, for instance by light or heat. Thus, using the method according to the invention, a variety of composites with different properties can be formed, as desired.
Upon infiltration of the microstructure, the material of the infiltrant may react with the material of the microstructure. The reaction products, possibly together with parts of the material of the microstructure that are broken loose from the microstructure during infiltration, will be incorporated in the infiltrant material and may form part of the second material forming the continuous matrix.
Further advantageous embodiments of methods according to the invention are described in the description of the drawings.
The invention further relates to a composite material comprising ceramic and/or relatively brittle metal particles embedded in a continuous matrix of a second material, obtainable according to a method of the invention, and a product manufactured from such a composite material.
The composite material is distinguished from known composites in its microstructure, by the presence of a large number of brittle fracture surfaces separated from each other by narrow channels, jointly forming a fine-mesh network of narrow channels, of an average diameter of typically a few hundred nanometers. Preferably, the average diameter of the channels ranges from 100 to 1000 nm, more preferably from 100 to 500 nm. The microstructure, after shock-wave compaction, will preferably comprise substantially no closed porosity. The volume percent of ceramic or other brittle material is then between 95 and 50 and preferably between 90 and 70.